Chlorine dioxide is an oxidation agent, and an important use thereof is as a bleaching agent in the cellulose industry, where it is the most common bleaching agent in the final stages.
There is a general trend to an increased use of chlorine dioxide at the expense of other bleaching agents, especially hypochlorite and chlorine. One can, i.a., note an increased admixture of chlorine dioxide also in the introductory stage, favorable synergistic effects being obtained.
Since cellulose plants use large amounts of sodium hydroxide for washing, and since the preparation of chlorine is connected to the preparation of sodium hydroxide through the chlorine alkali process, the reduced use of chlorine involves either a risk of a deficit of sodium hydroxide or an increased price thereof.
Normally chlorine dioxide is prepared by reduction of chlorate, and the most common processes for this can be summarized in the following gross formulas: EQU 2 NaClO.sub.3 + So.sub.2 .fwdarw.2 ClO.sub.2 + Na.sub.2 SO.sub.4 I. EQU (the Mathieson process) EQU 2 NaClO.sub.3 + CH.sub.3 OH + H.sub.2 SO.sub.4 .fwdarw.2 ClO.sub.2 + HCOOH + H.sub.2 O + Na.sub.2 SO.sub.4 II. EQU (the Solvey process) EQU NaClO.sub.3 + NaCl + H.sub.2 SO.sub.4 .fwdarw.ClO.sub.2 + 1/2 Cl.sub.2 + H.sub.2 O + Na.sub.2 SO.sub.4 III. EQU (the Rapson R-2-process, see the Canadian patent specification No. 543 589)
Thus, the reducing agent in these processes is sulphur dioxide, methanol and chloride ion respectively. Other reducing agents, such as chromic acid or nitrogen oxides have also been tested, but principally due to their higher prices they have not been commercially utilized to a considerable degree.
All these processes take place with an excess of a strong acid, usually sulphuric acid, and therefore the spent liquor of the reactor will consist of sodium sulphate in strong sulphuric acid or, if desired, sodium hydrogen sulphate in strong sulphuric acid.
It is essential from an economical as well as environmental point of view that this liquor be utilized.
Previously this liquor has been disposed of in the sewage system. However, the most usual thing has been to use it as "make up" in the cellulose plant, but in that case its content of sulphuric acid has not been utilized and chemicals have been necessary for its neutralization. Sometimes its high content of sulphuric acid has made it useful for other purposes, e.g., in the plant for resin cooking.
Increasingly rigorous environmental demands and rules for the washing of gases from boilers and evaporators and increased closing of the drainage systems of the plants have in many cases reduced the need of make up so much that the reactors will produce an excess of sodium sulphate and sulphuric acid which can not be disposed of in the sewage system.
Therefore great efforts have been made to make use of the liquor in another way.
By using a combined reactor/evaporator the sulphuric acid can be retained in the reactor and only solid sodium sulphate be withdrawn, i.e., the least possible amount of by-product for this process (the Rapson R-3-process, see the Swedish patent specification No. 312 789).
By replacing the addition of sodium chloride and part of the addition of sulphuric acid with hydrochloric acid, which, however, is a more expensive chemical substance than sulphuric acid, the produced amount of sodium sulphate can be additionally reduced.
However, these processes still produce the difficultly usable sodium sulphate, and therefore it has been suggested to convert this into sodium chloride and sulphuric acid: EQU Na.sub.2 SO.sub.4 + 2 HCl.fwdarw.2 NaCl + H.sub.2 SO.sub.4 IV. EQU (the Rapson R-4-process)
Thus this process requires another reactor system after the chlorine dioxide reactor to recover sulphuric acid, and then the sodium chloride product is still not made useful.
A similar result, i.e., a sodium chloride containing liquor, is obtained if the chlorate reduction is carried out merely with hydrochloric acid: EQU NaClO.sub.3 + 2 HCl.fwdarw.ClO.sub.2 + 1/2 Cl.sub.2 + H.sub.2 O + NaCl V.
if in this process, like in the previous ones, one tries to achieve a high degree of conversion of the chlorate by means of a high acid content, some problems with the following side reactions will occur: EQU NaClO.sub.3 + 6 HCl.fwdarw.3 Cl.sub.2 + NaCl + 3 H.sub.2 O EQU (see the Canadian patent specification No. 920 773)
which will increase with increasing concentration of chloride ion, and partly with the spent liquor, which due to the high price of hydrochloric acid must not go to waste or due to its acidity cannot be economically worked up to chlorate in an electrolytic cell, either, as the solution must first be electrolytically neutralized.
Therefore processes of this kind (see the Swedish patent specifications Nos. 155 759 and 337 007) operate with a low acid content, which permits electrolytic regeneration of the liquor to chlorate. In return these processes require a long residence time for the reaction between chlorate and hydrochloric acid and, consequently, several and big reactors. A high temperature is also used to increase the conversion rate, which beings increased risks of explosion, the mastering of which requires an increased number of apparatuses and process-technical compromises.
Thus, to sum up, both purely chemical regeneration trials by the addition of reagents as well as regeneration trials by electrolysis of residual solutions from chlorine dioxide reactors have so far caused problems seriously restricting the usefulness of the bleaching chemical agent chlorine dioxide, which is very valuable per se. Problems have arisen both in acidification with sulphuric acid and with hydrochloric acid, which acids have so far been predominant.